Help choosing the right propeller

What I have gathered from various threads is that in propellers, the diameter is linked with torque(thrust) and pitch is linked with speed. Now, motors have a a recommended propeller size that should be used for the most efficient setup. I have a few questions that I am not getting the answers to.. please help me out:

1) Suppose the max prop size for a motor on 3s is 7x4. Can we go to a 8x3 without putting undue stress on the motor?

2) Will a 7x4 and a 7x3.5 prop provide the same thrust when spun by identical motors on identical batteries with only difference in the top speed and power consumed?

You can go with the biggest or fastest prop within your motors limits or use the most efficient setup - least amp draw to power ratio.
Play around with the calcs and see what happens with prop and voltage change.

While calcs are good, the most accurate way to test is with a wattmeter. When you change pitch only, like your 7x3.5 and 7x4 example, thrust will differ slightly as the rpm will also be slightly higher on the lesser of the two pitches. This off course also depends on the kv of the motor, as to how big the difference will be.

What I have gathered from various threads is that in propellers, the diameter is linked with torque(thrust) and pitch is linked with speed.

Mostly, yes, but don't fall into the trap where you equate thrust with torque. I believe this comes from thinking of it in car terms, where torque is synonymous with the force pushing the car forward. In planes, that force is thrust, and only thrust, and not torque. The reason this distinction is important is because "torque" is also a factor in power systems, but it's entirely separate from the force driving the plane forward.

Quote:

1) Suppose the max prop size for a motor on 3s is 7x4. Can we go to a 8x3 without putting undue stress on the motor?

The diameter and pitch values, contrary to popular advice, are not interchangeable. An increase in diameter will generally put more strain on the motor than an equivalent measure of increase in pitch, so an 8x3 would most likely be borderline or above the motor's max. Different brands and prop designs will have different performance characteristics, though, so it's impossible to say anything definite based on the pitch and diameter numbers alone.

Quote:

2) Will a 7x4 and a 7x3.5 prop provide the same thrust when spun by identical motors on identical batteries with only difference in the top speed and power consumed?

Thrust is a tricky beast. In static testing under these conditions, the props will make around the same amount of thrust. Static thrust and dynamic thrust are different, though. The reason pitch affects speed is because it determines thrust at a given airspeed for a given RPM level. More pitch for a given airspeed (and flying relatively close to pitchspeed) will mean more thrust, which helps to overcome more drag and make the plane fly faster before thrust and drag are in equilibrium again.

The extreme example is something like this: If we spin both props up to 10,000 RPM, the pitchspeed of the 7x3.5 is about 33mph while that of the 7x4 is around 38mph. If we move both props at 33mph (assuming pitch figures are for actual aerodynamic pitch), the 7x3.5 will make zero thrust while the 7x4 will continue to make a little.

Thanks guys! Your posts really helped clear my doubts. Now for a more specific question..
I have this motor: D2822/17 Brushless Outrunner 1100kv for my quad.
(http://www.hobbyking.com/hobbyking/s...dProduct=18967)
It is producing not enough thrust with a gws 7x3.5 prop(hover at almost full throttle). I am running 4 of these on F20A escs and 20C 3.2Ah battery. The motor remains cool. What prop should I go for next? I do not need much speed but a little more thrust. I do not have a wattmeter

Thanks guys! Your posts really helped clear my doubts. Now for a more specific question.... I do not have a wattmeter

I just took delivery of a wattmeter (and I'm now waiting on a rev counter) because I was having your kind of prop questions. Also I ruined a motor 'testing' different props.

Already I've discovered you can't rely on specs given for some motors! E.g. I have motor which is supposed to run at peak efficiency with 5.5A, but it gets way too hot at 4.5A with the recommended 7x5 prop!

So, your asking for trouble runing without a wattmeter to keep an eye on things...

At 38g and 1100Kv, that motor's specs put it in the same class as a Suppo 2208/17 (39g, 1100Kv), and for all intents and purposes I'd say performance should be close to identical.

Dr. Kiwi's test results for the Suppo motor show a little over 10.5A with a GWS 9x5 prop at 11V input. For your setup, I'd suggest GWS RD9047 props. They'll probably draw a bit more current than the HD9050 from Phil's tests due to their bigger slow-flyer blades, but you shouldn't need anywhere near full throttle for most flying. Your ESCs have enough headroom to avoid issues with extended partial throttle, so everything looks good.

Whoa that was quick Maybe I should first go with 8x4 props and check if it generates the required thrust? Since it is below 20A I can use my multimeter to check the amp draw.. Should I stay below the rated ampere draw of 10A?

Staying below the 10A rating would be ideal, but it's not super-strictly necessary in this case since you won't be using full throttle very much.

If you're gonna go with smaller props, the RD8043s will give you a little more thrust than the regular HD8040s while drawing less current than the RD9047s, so there's sort of a nice middle ground there. I suspect the HD7035s are putting you around 5A right now at full throttle, so you definitely have room to expand.

What I have gathered from various threads is that in propellers, the diameter is linked with torque(thrust) and pitch is linked with speed. Now, motors have a a recommended propeller size that should be used for the most efficient setup. I have a few questions that I am not getting the answers to.. please help me out:

1) Suppose the max prop size for a motor on 3s is 7x4. Can we go to a 8x3 without putting undue stress on the motor?

2) Will a 7x4 and a 7x3.5 prop provide the same thrust when spun by identical motors on identical batteries with only difference in the top speed and power consumed?

Thanks

This is close.

The pitch is linked to speed, but the size of the prop is limited to the motor. If you calculate RPM and pitch, you will get pitch speed which is near the speed of the plane.

Your prop tips should not go above 600 mph.

1. Yes, but it will not be as good because the RPM of the motor does not change.

The pitch is linked to speed, but the size of the prop is limited to the motor. If you calculate RPM and pitch, you will get pitch speed which is near the speed of the plane.

Your prop tips should not go above 600 mph.

1. Yes, but it will not be as good because the RPM of the motor does not change.

2. Almost.

We have the tendency to speak of a certain size prop as if only the diameter and pitch make any difference at all. That is miles from the truth.

Take three 7x4 props from three different companies and you will find a WIDE range of differences in thrust, amp draw and RPM. Even more important than the numbers is the inherent qualities of the prop.

When the University of Illinois did a study of 140 props designed and sold for RC models they found a huge difference in quality, with props varying in efficiency coefficients from an abysmal .28 to about .65. This means that in the worst case, you could have two 7x4 props, one performing twice as good as the other. Their thrust, amp draw and RPM would be VERY different.

Without specifying the brand and model number of the prop along with its diameter and pitch, you just aren't saying very much at all. If your calculator does not include brand and model information then your calculator is definitely wrong in its results. Real results are based on real tests.

That's why I use Drive Calculator. It has a huge database of batteries, ESCs, motors and props by brand and model number. Of course huge isn't really big enough. But if you decide to stay with their tested results, you'll have a giant selection of options to build with confidence. Unfortunately the Turnigy D2822/17 1100kv motor isn't on their list.

I would resist the temptation to just substitute a similar motor in the calculations and try to determine results from that. Really there is no need. Just purchase motors that ARE on the list. MotoCalc works the same way with a similar sized list of actually tested components.

In spite of their lack of completeness, these programs are the only way I'd design a system. I'll let Dr Kiwi and others do the testing and I'll reap the benefits of their valuable contribution with one of those two programs: Drive Calculator (free program) or MotoCalc (commercial program).

I would ignore the free calculators online, they are a waste of time. Just get several props, setup a thrust stand and calculate how much thrust you get per watt on your watt meter on each prop. Select the prop with the best speed you want and the best thrust-watt.

I would ignore the free calculators online, they are a waste of time. Just get several props, setup a thrust stand and calculate how much thrust you get per watt on your watt meter on each prop. Select the prop with the best speed you want and the best thrust-watt.

Props are props, and our models are toys afer all.

Are you claiming that a normal hobbyist can duplicate the quality and skill of a Dr Kiwi and be willing to pay many times the cost of the models he flies to obtain information that has already been bought and paid for?

And what do you know of Drive Calculator? Specifically, what about it is worthy of saying it is "a waste of time" and that it should be ignored. Specifics please, as I have supplied specifics. I have gone well behind a blanket condemnation to supply information that is useful in selecting valid tools.

I designed the power system of my Slow Stick from information derived from Drive Calculator based on Dr Kiwi tests, using a combination not recorded anywhere as being used in that airplane before. The system worked almost identically as predicted, with RPM different by less than 50 RPM and thrust right on the money. I spent zero dollars and hit my perfect solution in one shot. How is your way superior? Tossing props in the trash is kinda wasteful! Building test equipment that has already been built better to develop inferior information is wasteful. I'd rather spend money on the stuff I fly. Just a personal preference there, and I may not be completely sane.

Are you claiming that a normal hobbyist can duplicate the quality and skill of a Dr Kiwi and be willing to pay many times the cost of the models he flies to obtain information that has already been bought and paid for?

And what do you know of Drive Calculator? Specifically, what about it is worthy of saying it is "a waste of time" and that it should be ignored. Specifics please, as I have supplied specifics. I have gone well behind a blanket condemnation to supply information that is useful in selecting valid tools.

I designed the power system of my Slow Stick from information derived from Drive Calculator based on Dr Kiwi tests, using a combination not recorded anywhere as being used in that airplane before. The system worked almost identically as predicted, with RPM different by less than 50 RPM and thrust right on the money. I spent zero dollars and hit my perfect solution in one shot. How is your way superior? Tossing props in the trash is kinda wasteful! Building test equipment that has already been built better to develop inferior information is wasteful. I'd rather spend money on the stuff I fly. Just a personal preference there, and I may not be completely sane.

Here are the specifics: it is a waste of time because you will spend lots of your free time playing with it when it is not very helpful. Like I mentioned, using a watt meter and extra propellers is perfectly correct. Propellers are cheap-most planes only have 2 or 5 possible propellers anyway, given that model plane propellers are manufactured in standard sizes. (11x8, 12x6, 12x8 - not 11.7x8, 11.9x5.5, et.)

I recommend this setup to everyone. A thrust stand and a watt meter-pick your desired pitch speed, and then use the prop with that speed and the greatest thrust-watt.